Followers

Principles of Plastering

Plastering is the
process of covering rough surfaces of walls, columns, ceilings and other building
components with thin coat of mortars to form a smooth durable surface. The
coating of mortar is termed as plaster.

Plastering
is done to achieve the following objects:

To protect the external
surfaces against penetration of rainwater and other atmospheric agencies.

To give smooth surface
in which dust and dirt cannot lodge.

To give decorative
effect.

To protect surfaces
against vermin.

To conceal inferior
materials or defective workmanship.

2
Requirements of good plaster

The plaster material
should fulfill the following requirements:

It should adhere to the
background, and should remain adhered during all variations in seasons and
other atmospheric conditions.

It should be hard and
durable.

It should possess good
workability.

It should be possible
to apply it during all weather conditions.

It should be cost
efficient.

It should effectively
check penetration of moisture.

3
Types of mortars for plastering

The selection of type
of plaster depends upon the following factors:

Availability of binding
materials.

Durability
requirements.

Finishing requirements.

Atmospheric conditions
and variations in weather.

Location of surface
(i.e. exposed surface or interior surfaces).

3.1
Cement mortar

Cement mortar is the
best mortar for external plastering work since it is practically nonabsorbent.It
is also preferred to lime plaster in both rooms etc., and in damp climates.
Cement mortar is much stronger than lime mortar. The mix proportion (i.e.
cement:sand) may vary from 1:4 to 1:6. Sand used for plastering should be
clean, coarse and angular.Cement plaster is applied either in two coats or in
three coats, the former being more common.For inferior work, single coat
plaster is sometimes provided.

6.4
Number of coats of plaster

The background over
which plastering is to be done depend upon the type of wall construction,such
as random rubble (R.R.) masonry, coarsed rubble masonry, brick masonry

4.1
Background No. of Coats

Stone work 3 or 2

Brick work or hollow
blocks 2 or 1

Concrete cast in situ 2
or 1

If plastering is done
in single coat only, its thickness should not exceed 12 mm nor should it be
less than 6 mm.

4.2
Two coat plaster

The following procedure
is adopted:

The background is
prepared by raking the joint to a depth of 20 mm, cleaning the surface and
well-watering it.

If the surface to be
plastered is very uneven, a preliminary coat is applied to fill up the hollows,
before the first coat.

The first coat or
rendering coat of plaster is applied, the thickness being equal to the specified
thickness of plaster less 2 to 3 mm. In order to maintain uniform thickness of
plaster, 15 cm x 15 cm size. Two dots are so formed in vertical line, at a
distance of about 2 m, and are plumbed by means of a plumb. A number of such
vertical screeds are formed at suitable spacing. Cement mortar is then applied
on the surface between the successive screeds and the surface is properly
finished. Before rendering hardens, it is suitably worked to provide mechanical
key for the final or finishing coat. The rendering coat is trowelled hard forcing
mortar into joints and over the surface. The rendering coat is kept wet for at
least 2 days, and then allowed to dry completely.

The thickness of final
or finishing coat may vary between 2 and 3 mm. Before applying the final coat,
the rendering coat is damped evenly. The final coat is applied with wooden floats
to a true even surface and finished with steel trowels. As far as possible, the
finishing coat should be applied starting from top towards bottom and completed
in one operation to eliminate joining marks.

4.3
Three coat plaster

The procedure for
applying three-coat plaster is similar to the two-coat plaster except that an intermediate
coat, known as floating coat is applied. The purpose of this coat of plaster is
to bring the plaster to an even surface. The thickness of rendering coat,
floating coat and finishing are kept 9 to 10 mm, 6 to 9 mm and 2 to 3 mm
respectively. The rendering coat is made rough.The floating coat is applied
about 4 to 7 days after applying the first coat. The finishing coat may be
applied about 6 hours after the application of floating coat.

4.4.
Single coat plaster

This is used only in
inferior quality work. It is applied similarly as two-coat plaster except that the
rendering coat, as applied for two-coat plaster, is finished off immediately
after it has sufficiently hardened.

5
Preparation of background

For plastering new
surfaces, all masonry joints should be raked to a depth of 10 mm in brick masonry
and 15 mm in stone masonry for providing key to the plaster. All mortar
droppings and dust, and laitance (in case of freshly laid concrete) should be
removed with the help of stiff wire brush. Any unevenness is levelled before
rendering is applied. For finish applied in three Principles of Construction
coats, local projections should not be more than 10 mm proud of general surface
and local depressions should not exceed 20 mm. For two-coat plaster, these
limitations are 5 mm and 10 mm respectively. The surface should be washed with
clean water and kept damp uniformly

to produce optimum
suction. In no case should the surface be kept so soaked that it causes the
green mortar to slide off, or so dry that it causes strong suction which
withdraws moisture from mortar and makes it weak, porous and friable. If
plaster is to be applied on old surface,all dirt, scool, oil, paint etc. should
be cleaned off. Loose and crumbling plaster layer should be removed to its full
thickness and the surface of the background should be exposed and joints properly
raked. The surface should be washed and kept damp to obtain optimum suction.

6
Recommended mortar mixes

7
Defects in plastering

The following defects
may arise in plasterwork:

7.1
Blistering of plastered surface

This is the formation
of small patches of plaster swelling out beyond the plastered surface,arising
out of late slaking of line particles in the plaster.

7.2
Cracking

Cracking consists of
formation of cracks or fissures in the plaster work resulting from the following
reasons:

Imperfect preparation
of background.

Structural defects in
building.

Discontinuity of
surface.

Movements in the
background due to its thermal expansion or rapid drying.

Movements in the
plaster surface itself, either due to expansion (in case of gypsum plaster) or
shrinkage (in case of lime sand plaster).

Excessive shrinkage due
to application of thick coat.

Faulty workmanship

7.3
Crazing

It is the formation of
a series of hair cracks on plastered surface, due to same reasons which cause
cracking.

7.4
Efflorescence

It is the whitish
crystalline substance that appears on the surface due to presence of salts in plaster-making
materials as well as building materials like bricks, sand, cement etc. and even
water. This gives a very bad appearance. It affects the adhesion of paint with
wall surface. Efflorescence can be removed to some extent by dry brushing and
washing the surface repeatedly.

7.5
Flaking

It is the formation of
very loose mass of plastered surface, due to poor bond between successive
coats.

7.6
Peeling

It is the complete
dislocation of some portion of plastered surface, resulting in the formation of
a patch. This also results from imperfect bond.

7.7
Popping

It is the formation of
conical hole in the plastered surface due to presence of some particles,which
expand on setting.